<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f1()">
<input type="button" value="减" onclick="f2()">
<input type="button" value="乘" onclick="f3()">
<input type="button" value="除" onclick="f4()">
<div>结果：</div>
</body>

<script>
  let i1 = document.querySelector("#i1")
  let i2 = document.querySelector("#i2")
  let d = document.querySelector("div")

  function f1() {
    if (isNaN(i1.value) || isNaN(i2.value)){
      d.innerText = "输入有误！！"
      return;
    }
    d.innerText = "结果："+(parseFloat(i1.value)+parseFloat(i2.value))
  }

  function f2() {
    if (isNaN(i1.value) || isNaN(i2.value)){
      d.innerText = "输入有误！！"
      return
    }
    d.innerText = "结果："+(parseFloat(i1.value)-parseFloat(i2.value))
  }

  function f3() {
    if (isNaN(i1.value) || isNaN(i2.value)){
      d.innerText = "输入有误！！"
      return
    }
    d.innerText = "结果："+(parseFloat(i1.value)*parseFloat(i2.value))
  }

  function f4() {
    if (isNaN(i1.value) || isNaN(i2.value)){
      d.innerText = "输入有误！！"
      return
    }
    d.innerText = "结果："+(parseFloat(i1.value)/parseFloat(i2.value))
  }
</script>

</html>